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  • Review Article
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Tn7: smarter than we thought

Nature Reviews Molecular Cell Biology volume 2, pages 806–814 (2001)Cite this article

Key Points

  • Transposons — a class of mobile genetic elements — are widespread in nature, being present in virtually every organism that has been examined. The bacterial transposon Tn7 is of particular interest because of its ability to use several different kinds of target sites. The highly evolved target-site selection pathways of Tn7 aid its dispersal among bacterial populations by directing insertions onto plasmids yet discourage any potentially harmful random insertions into bacterial chromosomes.

  • The Tn7 proteins TnsA, TnsB, and TnsC make up the core machinery for Tn7 transposition. TnsA and TnsB collaborate to form the transposase. TnsC is a regulator of transposase activity, communicating between the TnsAB transposase and the alternative target-site selecting proteins, TnsD and TnsE.

  • When the TnsE protein is utilized with the core TnsABC machinery, Tn7 preferentially directs insertions into conjugable plasmids, which can move between cells. Interaction between TnsE and a structure that is associated with lagging-strand DNA replication is probably crucial for target recognition in this pathway.

  • When the TnsD protein is utilized with the core machinery, insertions are directed into a single site in the bacterial chromosome, called attTn7. This site is highly conserved among bacterial chromosomes. TnsD specifically recognizes attTn7 and induces a distortion at the 5′ end of the binding site that is probably crucial in recruiting TnsC. Footprinting results indicate that TnsC could form a platform below the target DNA to receive and activate the TnsAB transposase to carry out recombination.

  • TnsC acts as a regulator of Tn7 transposition by its ability to interact both with the TnsAB transposase and the with target DNA that is bound by TnsD or TnsE. TnsC is also responsible for transposon immunity, a process that hinders insertions from occurring into a target DNA when a copy of Tn7 already resides in that DNA. TnsC might use a mechanism similar to the Ras molecular switch to balance the various "ON" and "OFF" signals that control transposition.

  • Future research with Tn7 should provide further insight into how multiprotein machines are assembled on DNA in replication, repair, and recombination.

Abstract

A notable feature of transposable elements — segments of DNA that can move from one position to another in genomes — is that they are highly prevalent, despite the fact that their translocation can result in mutation. The bacterial transposon Tn7 uses an elaborate system of target-site selection pathways that favours the dispersal of Tn7 in diverse hosts as well as minimizing its negative effects.

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Figure 1: Tn7 has two pathways of transposition to facilitate its safe propagation in diverse hosts.
Figure 2: The structure of Tn7.
Figure 3: TnsA and TnsB form a heteromeric transposase that excises the transposon through double-stranded DNA breaks.
Figure 4: TnsABC+E-mediated transposition occurs with an orientation bias in the direction of DNA replication.
Figure 5: TnsABC+E targets structures associated with lagging-strand DNA synthesis.
Figure 6: TnsD interacts with the attTn7 DNA sequence to recruit the TnsABC machinery to direct transposition into a single site in the bacterial chromosome.
Figure 7: Several similarities between target-site selection promoted by TnsD and triplex DNA implicate a crucial role for target DNA structure in transposition.

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Acknowledgements

We thank Z. Skelding and P. Kudavalli for comments and suggestions on the manuscript. The Howard Hughes Medical Institute, the National Institutes of Health and the National Science Foundation supported this work.

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Authors and Affiliations

  1. Department of Molecular Biology and Genetics, Howard Hughes Medical Institute, Johns Hopkins University School of Medicine, 725 North Wolfe Street, Baltimore, 21205, Maryland, USA

    Joseph E. Peters & Nancy L. Craig

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Corresponding author

Correspondence to Nancy L. Craig.

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DATABASES

GenBank:

attn7

Tn7-L

Tn7-R

 Swiss-Prot:

glmS

L29

Tn5468

TnsA

TnsB

TnsC

TnsD

TnsE

Glossary

REPLICON

A contiguous DNA segment with an origin(s) of DNA replication.

CONJUGAL PLASMIDS

A class of autonomously replicating circular DNA that can move between bacteria.

ORIGIN OF TRANSFER

A cis-acting DNA site found in conjugal plasmids that is recognized by the transfer proteins and at which DNA transfer initiates.

DISCONTINUOUS OR LAGGING-STRAND DNA SYNTHESIS

DNA replication that must continually be re-primed approximately every two kilobase pairs in bacteria.

ORIGIN OF BI-DIRECTIONAL DNA REPLICATION

The site at which the DNA strands are melted and the replisome is assembled for DNA replication, which progresses in the 3′ and 5′ directions and replicates both strands.

attTn7

A DNA sequence that is found in the highly conserved carboxy-terminal coding region of the glutamine synthetase (glmS) gene in bacteria and is recognized by the Tn7 protein, TnsD.

TRIPLEX DNA

A DNA structure in which a third DNA strand docks in the major groove of duplex DNA using atypical Hoogsteen hydrogen bonding.

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Peters, J., Craig, N. Tn7: smarter than we thought. Nat Rev Mol Cell Biol 2, 806–814 (2001). https://doi.org/10.1038/35099006

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